1. Field of the Invention
The present invention relates to a hydro-dynamic fluid bearing device, more detailedly, to a hydro-dynamic fluid bearing device of a spindle motor for an information system, an audio/visual system or the like, especially, to a hydro-dynamic fluid bearing device of a spindle motor suitable for an optical disk system or a magnetic disk system, and also to a method of making a bearing member used in such a device.
2. Related Background Art
Sliding bearings, ball bearings or hydro-dynamic fluid bearings are conventionally used in information systems such as LBPs (laser beam printers) and CD-ROM drive systems. Bearing devices in DVD (digital video disk) systems, which are new information systems, are planned to employ such bearings and some of them are practically utilized.
Bearing devices for laser beam printers, CD-ROM drive systems or the like, require high rigidity, low friction and good durability.
However, conventional bearing devices have the following problems.
In a bearing device using a sliding bearing, a thrust bearing is required in addition to a radial bearing. The number of parts thus increases. Besides, shaft run-out is large. Such shaft run-out is apt to occur in accordance with the size of the gap between a radial bearing and a shaft. Furthermore, wear resistance is bad. Abrasion is apt to be heavy in particular when the rotational speed is high.
In a bearing device using a ball bearing, the ball bearing itself is expensive. Furthermore, rotation unevenness and vibration are apt to occur.
In a bearing device using a hydro-dynamic fluid bearing (made of metal), a thrust bearing is required in addition to a radial bearing. The number of parts thus increases. Besides, wear resistance is low because abrasion is easy to occur due to contact at the time of start or stop. Furthermore, the manufacturing cost is high because of the formation of grooves for generating hydro-dynamic fluid and the highly accurate finish of a bearing surface.
Considering the above problems, the present inventor et al. proposed a hydro-dynamic fluid bearing using a bearing member made of resin which is superior in anti-friction and wear resistance, and can be formed in one body by injection molding, and the cost of which is low. Such a bearing device, however, has the following new problem. Since the rigidity of resin is lower than that of metal, the bearing member made of resin is displaced (elastically deformed) when unbalance quantity (radial load) is large.
In recent years, the rotational speeds of fluid bearing devices for spindle motors of optical disk systems or magnetic disk systems are tend to increase because of the demand of the high speed transmission of data. In a supporting bearing of such a spindle motor, the influence of centrifugal force at a high speed rotation due to the unbalance of a rotation member becomes larger.
FIG. 12 shows a cross-sectional view of a prior art bearing device. A rotation member comprises a shaft 130, a disk attachment flange 131 and a rotor 133. The shaft 130 is rigidly inserted in the disk attachment flange 131. The rotor 133 is fixed to the lower surface of the disk attachment flange 131.
A support member for supporting the shaft 130 comprises a stator 134, a base 135, a bearing member 136 and a steel ball 137. For operating as a radial hydro-dynamic fluid bearing, grooves for generating hydro-dynamic fluid are formed in a cylindrical radial bearing surface 136a of the bearing member 136. The bearing member 136 is firmly inserted in the base 135. The steel ball 137 is tightly inserted in the lower end portion of the bearing member 136. The steel ball 137 operates as a thrust bearing. The stator 134 is firmly inserted in and fixed to the bearing member 136.
The operation will be described. When the stator 134 is electrified, a rotating magnetic field is generated. The rotor 133 thereby rotates together with the shaft 130 and the disk attachment flange 131. The pressure of a lubricant in a radial bearing gap thereby increases because of a pumping effect by the grooves for generating hydro-dynamic fluid formed in the radial bearing surface 136a. The rotor 133 thus rotates in non-contact state between the radial bearing surface 136a and a radial receiving surface 136b which were initially in contact with each other.
In the thrust bearing, a sliding bearing is formed by point contact between a thrust bearing surface 130a of an end surface of the shaft 130 and a thrust receiving surface 137a. The lubricant is disposed between the thrust bearing surface 130a and the thrust receiving surface 137a. The rotor 133 thus rotates in point contact state through the lubricant.
Synthetic oils having good boundary lubrication properties were studied for such a lubricant. Particularly, load capacities of the above radial and thrust bearings are in proportion to the viscosity of a lubricant used. Since the change of the viscosity of synthetic oil with the change of temperature are large, an oil which meets the necessary load capacity at a high temperature, largely increases in its viscosity at a low temperature so as to increase the dynamic torque of a bearing. Contrarily, if an oil having the viscosity where the optimum dynamic torque of a bearing is obtained at a low temperature, is chosen, the viscosity decreases at a high temperature so that the load capacity becomes insufficient. Because synthetic oils are inferior in their temperature-viscosity properties in general, the diameter of the shaft was 1.5 mm for lowering the torque of the bearing device, or the radial bearing gap between the radial bearing surface 136a and the radial receiving surface 136b was narrowed to 3 .mu.m for insuring the necessary load capacity.
When the rotational speed of a bearing device becomes higher, however, the centrifugal force becomes larger due to the unbalance at the time of mounting a disk. The flexural rigidity of the shaft thus lacks, causing a problem that the run-out range of a rotational body becomes larger. Besides, in bearing devices, it is required to lower the torque at a low temperature because of a demand for saving the electric power to the device.
As another prior art, a dynamic air pressure bearing having a construction schematically shown in FIG. 13 is, used in a scanner motor for polygon mirror in a laser printer which is an information system. A shaft 202, in the outer surface of which grooves 203 for generating hydro-dynamic fluid are formed, is inserted in a sleeve 201 which is a cylindrical member. A radial dynamic air pressure bearing for supporting the sleeve 201 in the radial direction to the shaft 202 is formed by utilizing an air pressure which is generated by the grooves 203 for generating hydro-dynamic fluid at the relative rotation of the sleeve 201 and the shaft 202. An end opposite to an end through which the shaft 202 is inserted, is closed with a thrust plate 204. A pair of permanent magnets 205 and 206 is mounted on the end surface of the shaft 202 and the inner surface of the thrust plate 204 opposite to the former, respectively, so as to repel each other. A thrust magnetic bearing for supporting the sleeve 201 in the axial direction to the shaft 202 is formed by the repulsion between the permanent magnets 205 and 206.
In the dynamic air pressure bearing as shown in FIG. 13, however, because of the construction where air of low viscosity and little lubrication is used as a lubricant fluid, it is required to finish in very high accuracy the bearing surfaces such as the inner surface of the sleeve 201 and the outer surface of the shaft 202. Besides, the good slidability of those bearing surfaces must be insured. For these purposes, after the inner surface of the sleeve 201 made of structural steel is ground or honed, the inner surface is coated with a composite plating in which polyethylene fluoride resin such as Teflon (trade name), that is, polytetrafluoroethylene is impregnated in nickel. The inner surface is again ground or honed to insure the dimensional accuracy. On the other hand, the grooves for generating hydro-dynamic fluid must be formed by etching in the outer surface of the shaft 202 made of stainless steel which cooperates with the sleeve 201.
As for the sleeve 201, because a thick plating can not be formed, two times of grinding or honing are required. There are problems that the manufacturing cost increases in addition to increasing the cost for plating. As for the shaft 202, there are problems that the process of etching is complex and has need of a long time and the cost increases. Since the magnetic bearing using the repulsion between the permanent magnets 205 and 206 is employed for the thrust bearing, there are problems that the construction is complex, the number of parts increases and the cost for manufacturing the whole of the bearing is high.